Extensible execution unit interface architecture

ABSTRACT

A method and circuit arrangement tightly couple together decode logic associated with multiple types of execution units and having varying priorities to enable instructions that are decoded as valid instructions for multiple types of execution units to be forwarded to a highest priority type of execution unit among the multiple types of execution units. Among other benefits, when an auxiliary execution unit is coupled to a general purpose processing core with the decode logic for the auxiliary execution unit tightly coupled with the decode logic for the general purpose processing core, the auxiliary execution unit may be used to effectively overlay new functionality for an existing instruction that is normally executed by the general purpose processing core, e.g., to patch a design flaw in the general purpose processing core or to provide improved performance for specialized applications.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/766,508, filed on Feb. 13, 2013 by Adam J. Muff et al., the entiredisclosure of which is incorporated by reference herein.

FIELD OF THE INVENTION

The invention is generally related to data processing, and in particularto processor architectures and execution units incorporated therein.

BACKGROUND OF THE INVENTION

Computers and other programmable electronic devices continue to beintegrated into every aspect of society. Whereas computers once occupiedentire rooms, computers and the integrated circuit devices, or chips,that power such computers have now been miniaturized and commoditized tosuch an extent that they can be found in even the most mundane ofproducts and devices.

Programmable chips such as microprocessors, microcontrollers,Application Specific Integrated Circuits (ASIC's) and the like continueto increase in complexity and power while costs, power consumption andfeature sizes decrease. Whereas computers once incorporated separatechips for processors, graphics controllers, coprocessors, memorycontrollers, and other chipset logic, oftentimes all of these types offunctional units are integrated into a single programmable chip.Moreover, once a particular chip design has been developed, tested andverified, manufacturing costs are often comparatively low on a per unitbasis.

A significant portion of the effort and expense associated with bringinga programmable chip to market are therefore devoted to the initialdesign, testing and verification of a programmable chip. For thisreason, design reuse is employed whenever possible so that portions of aprogrammable chip, such as particular processor core designs, functionalunits, and other logic blocks, which have previously been designed,tested and verified, do not need to be recreated from scratch.

Even with design reuse, however, the integration of multiple designcomponents into a common design frequently requires some custom logic tobe designed and tested to ensure that the components will correctlyoperate with one another once integrated onto the same programmablechip. For example, modern microprocessors typically include functionalunits such as issue or instruction units, load/store units, executionunits, memory controllers, graphics controllers, cache and otheron-board memories, etc., and development of such microprocessors oftenrequires substantial development, testing and verification efforts toensure that all of the functional units operate in an intended manner.

In addition, microprocessor designs may be utilized for different typesof workloads, and it has been found that different types of workloadsare often handled most efficiently using different types ofinstructions. General purpose office software, for example, typicallyrelies primarily on fixed point instructions, while collision detectionphysics algorithms typically used in computer gaming are mostefficiently performed using floating point scalar instructions. Highspeed 3D graphics algorithms like rasterization are most efficientlyperformed using vector fixed point instructions while having very littleneed for double precision vector floating point instructions.Nonetheless, most workloads also usually require some basic instructionsat least for basic control flow, e.g., instructions for performingbranches, loads/stores, and fixed point math.

As a result, many microprocessor designs rely on a basic design that canbe extended through the use of different functional units to providevarious designs that are optimized for different applications. Forexample, it may be desirable to extend the functionality of amicroprocessor by integrating multiple processing cores together tofacilitate parallel processing, as well as integrating various executionunits within a processing core to optimize the core to handle certaintypes of workloads. For example, while a basic microprocessor designoften includes a processing core with an issue or instruction unit thatissues instructions to an execution unit referred to as a fixed pointunit, integer unit or arithmetic logic unit that handles integeroperations, additional execution units, e.g., floating point executionunits, graphics engines, physics engines, encryption engines, and thelike may be incorporated into the basic design to provide an applicationspecific design that is optimized for certain applications.

With the ability to combine different types of execution units invarious processing cores and programmable chips, significant designflexibility is provided for developing application-specific hardware. Onthe other hand, given the substantial up-front efforts still required todesign, test and verify multiple functional units integrated intoapplication-specific programmable chips, there are still substantialcosts associated with bringing application-specific programmable chipsto market.

Therefore, a significant need continues to exist in the art for a mannerof facilitating the development of application-specific programmablechips and electronic devices incorporating the same.

SUMMARY OF THE INVENTION

The invention addresses these and other problems associated with theprior art by providing a method and circuit arrangement that utilize anextensible execution unit interface architecture that tightly couplestogether decode logic associated with multiple types of execution unitsand having varying priorities to enable instructions that are decoded asvalid instructions for multiple types of execution units to be forwardedto a highest priority type of execution unit among the multiple types ofexecution units. For example, decode logic respectively associated withfirst and second types of execution units may be coupled together andconfigured to concurrently receive and decode the same instructions inan instruction stream. Whenever an instruction is decoded as a validinstruction for only one of the types of execution units, theinstruction may be forwarded to the corresponding type of executionunit. However, when the instruction is decoded as a valid instructionfor both types of execution units, the relative priorities of thedifferent types of execution units may be used to select a highestpriority type of execution unit to execute the instruction.

Among other benefits, when an auxiliary execution unit is coupled to ageneral purpose processing core with the decode logic for the auxiliaryexecution unit tightly coupled with the decode logic for the generalpurpose processing core, the auxiliary execution unit may be used toeffectively overlay new functionality for an existing instruction thatis normally executed by the general purpose processing core. Thus, forexample, in a design flaw exists in the general purpose processing corethat causes a sub-optimal execution of a particular instruction, anauxiliary execution unit may be designed to correct the design flaw, andthe decode logic for the auxiliary execution unit may be configured todecode that instruction as a valid instruction, such that whenever theinstruction is decoded, the auxiliary execution unit will execute theinstruction in lieu of the general purpose processing core. Also, inspecialized applications where higher performance is desired for certaininstructions, a specialized auxiliary execution unit may be designed tooptimize execution of those instructions, and often avoiding the need tomodify the general purpose processing core to accommodate theperformance concerns of the specialized application.

Therefore, consistent with one aspect of the invention, a circuitarrangement includes first decode logic associated with a first type ofexecution unit and configured to receive an instruction and decode theinstruction, and second decode logic associated with a second type ofexecution unit and configured to receive the instruction and decode theinstruction. The instruction is concurrently forwarded to the first andsecond decode logic, and the first decode logic has a higher prioritythan the second decode logic such that if the first and second decodelogic respectively decode the instruction as a valid instruction for thefirst and second types of execution unit, the instruction is executed bya first execution unit of the first type of execution unit rather asecond execution unit of the second type of execution unit.

Consistent with another aspect of the invention, a method is providedfor patching a processing unit design, where the processing unit designdefines a core including first decode logic and a first execution unit,where the first decode logic is configured to decode an instruction forthe first execution unit and forward the instruction to the firstexecution unit for execution thereby, and where the core includes adesign flaw that causes the first execution unit to execute theinstruction with sub-optimal performance. The method includes adding asecond execution unit to the processing unit design, where the secondexecution unit is configured to execute the instruction with greaterperformance than the first execution unit such that the second executionunit corrects for the design flaw in the core, and adding second decodelogic to the processing unit design, where the second decode logic isconfigured to receive the instruction concurrently with the first decodelogic and decode the instruction, and where the second decode logic isconfigured to, in response to decoding the instruction as a validinstruction for the second execution unit, assert an invalidate signalto the first decode logic to disable forwarding of the instruction bythe first decode logic to the fixed point execution unit and forward theinstruction to the second execution unit for execution thereby.

These and other advantages and features, which characterize theinvention, are set forth in the claims annexed hereto and forming afurther part hereof. However, for a better understanding of theinvention, and of the advantages and objectives attained through itsuse, reference should be made to the Drawings, and to the accompanyingdescriptive matter, in which there is described exemplary embodiments ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of exemplary automated computing machineryincluding an exemplary computer useful in data processing consistentwith embodiments of the present invention.

FIG. 2 is a block diagram of an exemplary NOC implemented in thecomputer of FIG. 1.

FIG. 3 is a block diagram illustrating in greater detail an exemplaryimplementation of a node from the NOC of FIG. 2.

FIG. 4 is a block diagram illustrating an exemplary implementation of anIP block from the NOC of FIG. 2.

FIG. 5 is a block diagram illustrating an exemplary implementation of anIP block from the NOC of FIG. 2, and configured to implement anextensible execution unit interface architecture consistent with theinvention.

FIG. 6 is a priority table for use in the IP block of FIG. 5.

FIG. 7 is an alternate priority table to that of FIG. 6, and definingpriorities for three types of execution units.

FIG. 8 is a block diagram illustrating a processing unit designintegrating a general purpose processing core with an auxiliaryexecution unit using an extensible execution unit interface architectureconsistent with the invention.

DETAILED DESCRIPTION

Embodiments consistent with the invention implement an extensibleexecution unit interface architecture that tightly couples togetherdecode logic associated with multiple types of execution units andhaving varying priorities to enable instructions that are decoded asvalid instructions for multiple types of execution units to be forwardedto a highest priority type of execution unit among the multiple types ofexecution units. In the embodiments discussed hereinafter, for example,decode logic respectively associated with first and second types ofexecution units may be coupled together and configured to concurrentlyreceive and decode the same instructions in an instruction stream.Whenever an instruction is decoded as a valid instruction for only oneof the types of execution units, the instruction may be forwarded to thecorresponding type of execution unit. However, when the instruction isdecoded as a valid instruction for both types of execution units, therelative priorities of the different types of execution units may beused to select a highest priority type of execution unit to execute theinstruction.

An extensible execution unit interface architecture may be implemented,for example, in a general purpose processing unit, which within thecontext of the invention may include practically any type of processingunit, e.g., as disposed in an IP block, a processor chip, a processorcore, etc., and capable of executing instructions to handle generalpurpose workloads. While a general purpose processing unit may includevarious components, execution units or accelerators that optimize oraccelerate particular types of workloads, e.g., image processing,graphics, scientific workloads, transaction processing, etc., a generalpurpose processing unit is otherwise capable of handling multiple typesof workloads, even if it does so sub-optimally as compared to a morespecialized processing unit.

In addition, an extensible execution unit interface architecture may besuitable for integrating an auxiliary execution unit (AXU) with ageneral purpose processing unit, e.g., to address a design flaw or toaugment the capabilities of the general purpose processing unit. In thisregard, an auxiliary execution unit may be configured as a scalar orvector fixed point execution unit (XU), a scalar or vector floatingpoint execution unit (FPU), or various types of accelerators orspecialized execution units (e.g., encryption/decryption engines, DMAengines, compression/decompression engines, physics engines, graphicsprocessors, coprocessors, etc.).

Other variations and modifications will be apparent to one of ordinaryskill in the art. Therefore, the invention is not limited to thespecific implementations discussed herein.

Hardware and Software Environment

Now turning to the drawings, wherein like numbers denote like partsthroughout the several views, FIG. 1 illustrates exemplary automatedcomputing machinery including an exemplary computer 10 useful in dataprocessing consistent with embodiments of the present invention.Computer 10 of FIG. 1 includes at least one computer processor 12 or‘CPU’ as well as random access memory 14 (‘RAM’), which is connectedthrough a high speed memory bus 16 and bus adapter 18 to processor 12and to other components of the computer 10.

Stored in RAM 14 is an application program 20, a module of user-levelcomputer program instructions for carrying out particular dataprocessing tasks such as, for example, word processing, spreadsheets,database operations, video gaming, stock market simulations, atomicquantum process simulations, or other user-level applications. Alsostored in RAM 14 is an operating system 22. Operating systems useful inconnection with embodiments of the invention include UNIX™, Linux™Microsoft Windows XP™, AIX™, IBM's i5/OS™, and others as will occur tothose of skill in the art. Operating system 22 and application 20 in theexample of FIG. 1 are shown in RAM 14, but many components of suchsoftware typically are stored in non-volatile memory also, e.g., on adisk drive 24.

As will become more apparent below, embodiments consistent with theinvention may be implemented within Network On Chip (NOC) integratedcircuit devices, or chips, and as such, computer 10 is illustratedincluding two exemplary NOCs: a video adapter 26 and a coprocessor 28.NOC video adapter 26, which may alternatively be referred to as agraphics adapter, is an example of an I/O adapter specially designed forgraphic output to a display device 30 such as a display screen orcomputer monitor. NOC video adapter 26 is connected to processor 12through a high speed video bus 32, bus adapter 18, and the front sidebus 34, which is also a high speed bus. NOC Coprocessor 28 is connectedto processor 12 through bus adapter 18, and front side buses 34 and 36,which is also a high speed bus. The NOC coprocessor of FIG. 1 may beoptimized, for example, to accelerate particular data processing tasksat the behest of the main processor 12.

The exemplary NOC video adapter 26 and NOC coprocessor 28 of FIG. 1 eachinclude a NOC, including integrated processor (‘IP’) blocks, routers,memory communications controllers, and network interface controllers,the details of which will be discussed in greater detail below inconnection with FIGS. 2-3. The NOC video adapter and NOC coprocessor areeach optimized for programs that use parallel processing and alsorequire fast random access to shared memory. It will be appreciated byone of ordinary skill in the art having the benefit of the instantdisclosure, however, that the invention may be implemented in devicesand device architectures other than NOC devices and devicearchitectures. The invention is therefore not limited to implementationwithin an NOC device.

Computer 10 of FIG. 1 includes disk drive adapter 38 coupled through anexpansion bus 40 and bus adapter 18 to processor 12 and other componentsof the computer 10. Disk drive adapter 38 connects non-volatile datastorage to the computer 10 in the form of disk drive 24, and may beimplemented, for example, using Integrated Drive Electronics (‘IDE’)adapters, Small Computer System Interface (‘SCSI’) adapters, and othersas will occur to those of skill in the art. Non-volatile computer memoryalso may be implemented for as an optical disk drive, electricallyerasable programmable read-only memory (so-called ‘EEPROM’ or ‘Flash’memory), RAM drives, and so on, as will occur to those of skill in theart.

Computer 10 also includes one or more input/output (‘I/O’) adapters 42,which implement user-oriented input/output through, for example,software drivers and computer hardware for controlling output to displaydevices such as computer display screens, as well as user input fromuser input devices 44 such as keyboards and mice. In addition, computer10 includes a communications adapter 46 for data communications withother computers 48 and for data communications with a datacommunications network 50. Such data communications may be carried outserially through RS-232 connections, through external buses such as aUniversal Serial Bus (‘USB’), through data communications datacommunications networks such as IP data communications networks, and inother ways as will occur to those of skill in the art. Communicationsadapters implement the hardware level of data communications throughwhich one computer sends data communications to another computer,directly or through a data communications network. Examples ofcommunications adapters suitable for use in computer 10 include modemsfor wired dial-up communications, Ethernet (IEEE 802.3) adapters forwired data communications network communications, and 802.11 adaptersfor wireless data communications network communications.

For further explanation, FIG. 2 sets forth a functional block diagram ofan example NOC 102 according to embodiments of the present invention.The NOC in FIG. 2 is implemented on a ‘chip’ 100, that is, on anintegrated circuit. NOC 102 includes integrated processor (‘IP’) blocks104, routers 110, memory communications controllers 106, and networkinterface controllers 108 grouped into interconnected nodes. Each IPblock 104 is adapted to a router 110 through a memory communicationscontroller 106 and a network interface controller 108. Each memorycommunications controller controls communications between an IP blockand memory, and each network interface controller 108 controls inter-IPblock communications through routers 110.

In NOC 102, each IP block represents a reusable unit of synchronous orasynchronous logic design used as a building block for data processingwithin the NOC. The term ‘IP block’ is sometimes expanded as‘intellectual property block,’ effectively designating an IP block as adesign that is owned by a party, that is the intellectual property of aparty, to be licensed to other users or designers of semiconductorcircuits. In the scope of the present invention, however, there is norequirement that IP blocks be subject to any particular ownership, sothe term is always expanded in this specification as ‘integratedprocessor block.’ IP blocks, as specified here, are reusable units oflogic, cell, or chip layout design that may or may not be the subject ofintellectual property. IP blocks are logic cores that can be formed asASIC chip designs or FPGA logic designs.

One way to describe IP blocks by analogy is that IP blocks are for NOCdesign what a library is for computer programming or a discreteintegrated circuit component is for printed circuit board design. InNOCs consistent with embodiments of the present invention, IP blocks maybe implemented as generic gate netlists, as complete special purpose orgeneral purpose microprocessors, or in other ways as may occur to thoseof skill in the art. A netlist is a Boolean-algebra representation(gates, standard cells) of an IP block's logical-function, analogous toan assembly-code listing for a high-level program application. NOCs alsomay be implemented, for example, in synthesizable form, described in ahardware description language such as Verilog or VHDL. In addition tonetlist and synthesizable implementation, NOCs also may be delivered inlower-level, physical descriptions. Analog IP block elements such asSERDES, PLL, DAC, ADC, and so on, may be distributed in atransistor-layout format such as GDSII. Digital elements of IP blocksare sometimes offered in layout format as well. It will also beappreciated that IP blocks, as well as other logic circuitry implementedconsistent with the invention may be distributed in the form of computerdata files, e.g., logic definition program code, that define at variouslevels of detail the functionality and/or layout of the circuitarrangements implementing such logic. Thus, while the invention has andhereinafter will be described in the context of circuit arrangementsimplemented in fully functioning integrated circuit devices, dataprocessing systems utilizing such devices, and other tangible, physicalhardware circuits, those of ordinary skill in the art having the benefitof the instant disclosure will appreciate that the invention may also beimplemented within a program product, and that the invention appliesequally regardless of the particular type of computer readable storagemedium being used to distribute the program product. Examples ofcomputer readable storage media include, but are not limited to,physical, recordable type media such as volatile and non-volatile memorydevices, floppy disks, hard disk drives, CD-ROMs, and DVDs (amongothers).

Each IP block 104 in the example of FIG. 2 is adapted to a router 110through a memory communications controller 106. Each memorycommunication controller is an aggregation of synchronous andasynchronous logic circuitry adapted to provide data communicationsbetween an IP block and memory. Examples of such communications betweenIP blocks and memory include memory load instructions and memory storeinstructions. The memory communications controllers 106 are described inmore detail below with reference to FIG. 3. Each IP block 104 is alsoadapted to a router 110 through a network interface controller 108,which controls communications through routers 110 between IP blocks 104.Examples of communications between IP blocks include messages carryingdata and instructions for processing the data among IP blocks inparallel applications and in pipelined applications. The networkinterface controllers 108 are also described in more detail below withreference to FIG. 3.

Routers 110, and the corresponding links 118 therebetween, implement thenetwork operations of the NOC. The links 118 may be packet structuresimplemented on physical, parallel wire buses connecting all the routers.That is, each link may be implemented on a wire bus wide enough toaccommodate simultaneously an entire data switching packet, includingall header information and payload data. If a packet structure includes64 bytes, for example, including an eight byte header and 56 bytes ofpayload data, then the wire bus subtending each link is 64 bytes wide,512 wires. In addition, each link may be bi-directional, so that if thelink packet structure includes 64 bytes, the wire bus actually contains1024 wires between each router and each of its neighbors in the network.In such an implementation, a message could include more than one packet,but each packet would fit precisely onto the width of the wire bus. Inthe alternative, a link may be implemented on a wire bus that is onlywide enough to accommodate a portion of a packet, such that a packetwould be broken up into multiple beats, e.g., so that if a link isimplemented as 16 bytes in width, or 128 wires, a 64 byte packet couldbe broken into four beats. It will be appreciated that differentimplementations may used different bus widths based on practicalphysical limits as well as desired performance characteristics. If theconnection between the router and each section of wire bus is referredto as a port, then each router includes five ports, one for each of fourdirections of data transmission on the network and a fifth port foradapting the router to a particular IP block through a memorycommunications controller and a network interface controller.

Each memory communications controller 106 controls communicationsbetween an IP block and memory. Memory can include off-chip main RAM112, memory 114 connected directly to an IP block through a memorycommunications controller 106, on-chip memory enabled as an IP block116, and on-chip caches. In NOC 102, either of the on-chip memories 114,116, for example, may be implemented as on-chip cache memory. All theseforms of memory can be disposed in the same address space, physicaladdresses or virtual addresses, true even for the memory attacheddirectly to an IP block. Memory addressed messages therefore can beentirely bidirectional with respect to IP blocks, because such memorycan be addressed directly from any IP block anywhere on the network.Memory 116 on an IP block can be addressed from that IP block or fromany other IP block in the NOC. Memory 114 attached directly to a memorycommunication controller can be addressed by the IP block that isadapted to the network by that memory communication controller—and canalso be addressed from any other IP block anywhere in the NOC.

NOC 102 includes two memory management units (‘MMUs’) 120, 122,illustrating two alternative memory architectures for NOCs consistentwith embodiments of the present invention. MMU 120 is implemented withinan IP block, allowing a processor within the IP block to operate invirtual memory while allowing the entire remaining architecture of theNOC to operate in a physical memory address space. MMU 122 isimplemented off-chip, connected to the NOC through a data communicationsport 124. The port 124 includes the pins and other interconnectionsrequired to conduct signals between the NOC and the MMU, as well assufficient intelligence to convert message packets from the NOC packetformat to the bus format required by the external MMU 122. The externallocation of the MMU means that all processors in all IP blocks of theNOC can operate in virtual memory address space, with all conversions tophysical addresses of the off-chip memory handled by the off-chip MMU122.

In addition to the two memory architectures illustrated by use of theMMUs 120, 122, data communications port 126 illustrates a third memoryarchitecture useful in NOCs capable of being utilized in embodiments ofthe present invention. Port 126 provides a direct connection between anIP block 104 of the NOC 102 and off-chip memory 112. With no MMU in theprocessing path, this architecture provides utilization of a physicaladdress space by all the IP blocks of the NOC. In sharing the addressspace bi-directionally, all the IP blocks of the NOC can access memoryin the address space by memory-addressed messages, including loads andstores, directed through the IP block connected directly to the port126. The port 126 includes the pins and other interconnections requiredto conduct signals between the NOC and the off-chip memory 112, as wellas sufficient intelligence to convert message packets from the NOCpacket format to the bus format required by the off-chip memory 112.

In the example of FIG. 2, one of the IP blocks is designated a hostinterface processor 128. A host interface processor 128 provides aninterface between the NOC and a host computer 10 in which the NOC may beinstalled and also provides data processing services to the other IPblocks on the NOC, including, for example, receiving and dispatchingamong the IP blocks of the NOC data processing requests from the hostcomputer. A NOC may, for example, implement a video graphics adapter 26or a coprocessor 28 on a larger computer 10 as described above withreference to FIG. 1. In the example of FIG. 2, the host interfaceprocessor 128 is connected to the larger host computer through a datacommunications port 130. The port 130 includes the pins and otherinterconnections required to conduct signals between the NOC and thehost computer, as well as sufficient intelligence to convert messagepackets from the NOC to the bus format required by the host computer 10.In the example of the NOC coprocessor in the computer of FIG. 1, such aport would provide data communications format translation between thelink structure of the NOC coprocessor 28 and the protocol required forthe front side bus 36 between the NOC coprocessor 28 and the bus adapter18.

FIG. 3 next illustrates a functional block diagram illustrating ingreater detail the components implemented within an IP block 104, memorycommunications controller 106, network interface controller 108 androuter 110 in NOC 102, collectively illustrated at 132. IP block 104includes a computer processor 134 and I/O functionality 136. In thisexample, computer memory is represented by a segment of random accessmemory (‘RAM’) 138 in IP block 104. The memory, as described above withreference to FIG. 2, can occupy segments of a physical address spacewhose contents on each IP block are addressable and accessible from anyIP block in the NOC. The processors 134, I/O capabilities 136, andmemory 138 in each IP block effectively implement the IP blocks asgenerally programmable microcomputers. As explained above, however, inthe scope of the present invention, IP blocks generally representreusable units of synchronous or asynchronous logic used as buildingblocks for data processing within a NOC. Implementing IP blocks asgenerally programmable microcomputers, therefore, although a commonembodiment useful for purposes of explanation, is not a limitation ofthe present invention.

In NOC 102 of FIG. 3, each memory communications controller 106 includesa plurality of memory communications execution engines 140. Each memorycommunications execution engine 140 is enabled to execute memorycommunications instructions from an IP block 104, includingbidirectional memory communications instruction flow 141, 142, 144between the network and the IP block 104. The memory communicationsinstructions executed by the memory communications controller mayoriginate, not only from the IP block adapted to a router through aparticular memory communications controller, but also from any IP block104 anywhere in NOC 102. That is, any IP block in the NOC can generate amemory communications instruction and transmit that memorycommunications instruction through the routers of the NOC to anothermemory communications controller associated with another IP block forexecution of that memory communications instruction. Such memorycommunications instructions can include, for example, translationlookaside buffer control instructions, cache control instructions,barrier instructions, and memory load and store instructions.

Each memory communications execution engine 140 is enabled to execute acomplete memory communications instruction separately and in parallelwith other memory communications execution engines. The memorycommunications execution engines implement a scalable memory transactionprocessor optimized for concurrent throughput of memory communicationsinstructions. Memory communications controller 106 supports multiplememory communications execution engines 140 all of which runconcurrently for simultaneous execution of multiple memorycommunications instructions. A new memory communications instruction isallocated by the memory communications controller 106 to a memorycommunications engine 140 and memory communications execution engines140 can accept multiple response events simultaneously. In this example,all of the memory communications execution engines 140 are identical.Scaling the number of memory communications instructions that can behandled simultaneously by a memory communications controller 106,therefore, is implemented by scaling the number of memory communicationsexecution engines 140.

In NOC 102 of FIG. 3, each network interface controller 108 is enabledto convert communications instructions from command format to networkpacket format for transmission among the IP blocks 104 through routers110. The communications instructions may be formulated in command formatby the IP block 104 or by memory communications controller 106 andprovided to the network interface controller 108 in command format. Thecommand format may be a native format that conforms to architecturalregister files of IP block 104 and memory communications controller 106.The network packet format is typically the format required fortransmission through routers 110 of the network. Each such message iscomposed of one or more network packets. Examples of such communicationsinstructions that are converted from command format to packet format inthe network interface controller include memory load instructions andmemory store instructions between IP blocks and memory. Suchcommunications instructions may also include communications instructionsthat send messages among IP blocks carrying data and instructions forprocessing the data among IP blocks in parallel applications and inpipelined applications.

In NOC 102 of FIG. 3, each IP block is enabled to sendmemory-address-based communications to and from memory through the IPblock's memory communications controller and then also through itsnetwork interface controller to the network. A memory-address-basedcommunications is a memory access instruction, such as a loadinstruction or a store instruction, that is executed by a memorycommunication execution engine of a memory communications controller ofan IP block. Such memory-address-based communications typicallyoriginate in an IP block, formulated in command format, and handed offto a memory communications controller for execution.

Many memory-address-based communications are executed with messagetraffic, because any memory to be accessed may be located anywhere inthe physical memory address space, on-chip or off-chip, directlyattached to any memory communications controller in the NOC, orultimately accessed through any IP block of the NOC—regardless of whichIP block originated any particular memory-address-based communication.Thus, in NOC 102, all memory-address-based communications that areexecuted with message traffic are passed from the memory communicationscontroller to an associated network interface controller for conversionfrom command format to packet format and transmission through thenetwork in a message. In converting to packet format, the networkinterface controller also identifies a network address for the packet independence upon the memory address or addresses to be accessed by amemory-address-based communication. Memory address based messages areaddressed with memory addresses. Each memory address is mapped by thenetwork interface controllers to a network address, typically thenetwork location of a memory communications controller responsible forsome range of physical memory addresses. The network location of amemory communication controller 106 is naturally also the networklocation of that memory communication controller's associated router110, network interface controller 108, and IP block 104. The instructionconversion logic 150 within each network interface controller is capableof converting memory addresses to network addresses for purposes oftransmitting memory-address-based communications through routers of aNOC.

Upon receiving message traffic from routers 110 of the network, eachnetwork interface controller 108 inspects each packet for memoryinstructions. Each packet containing a memory instruction is handed tothe memory communications controller 106 associated with the receivingnetwork interface controller, which executes the memory instructionbefore sending the remaining payload of the packet to the IP block forfurther processing. In this way, memory contents are always prepared tosupport data processing by an IP block before the IP block beginsexecution of instructions from a message that depend upon particularmemory content.

In NOC 102 of FIG. 3, each IP block 104 is enabled to bypass its memorycommunications controller 106 and send inter-IP block, network-addressedcommunications 146 directly to the network through the IP block'snetwork interface controller 108. Network-addressed communications aremessages directed by a network address to another IP block. Suchmessages transmit working data in pipelined applications, multiple datafor single program processing among IP blocks in a SIMD application, andso on, as will occur to those of skill in the art. Such messages aredistinct from memory-address-based communications in that they arenetwork addressed from the start, by the originating IP block whichknows the network address to which the message is to be directed throughrouters of the NOC. Such network-addressed communications are passed bythe IP block through I/O functions 136 directly to the IP block'snetwork interface controller in command format, then converted to packetformat by the network interface controller and transmitted throughrouters of the NOC to another IP block. Such network-addressedcommunications 146 are bi-directional, potentially proceeding to andfrom each IP block of the NOC, depending on their use in any particularapplication. Each network interface controller, however, is enabled toboth send and receive such communications to and from an associatedrouter, and each network interface controller is enabled to both sendand receive such communications directly to and from an associated IPblock, bypassing an associated memory communications controller 106.

Each network interface controller 108 in the example of FIG. 3 is alsoenabled to implement virtual channels on the network, characterizingnetwork packets by type. Each network interface controller 108 includesvirtual channel implementation logic 148 that classifies eachcommunication instruction by type and records the type of instruction ina field of the network packet format before handing off the instructionin packet form to a router 110 for transmission on the NOC. Examples ofcommunication instruction types include inter-IP blocknetwork-address-based messages, request messages, responses to requestmessages, invalidate messages directed to caches; memory load and storemessages; and responses to memory load messages, etc.

Each router 110 in the example of FIG. 3 includes routing logic 152,virtual channel control logic 154, and virtual channel buffers 156. Therouting logic typically is implemented as a network of synchronous andasynchronous logic that implements a data communications protocol stackfor data communication in the network formed by the routers 110, links118, and bus wires among the routers. Routing logic 152 includes thefunctionality that readers of skill in the art might associate inoff-chip networks with routing tables, routing tables in at least someembodiments being considered too slow and cumbersome for use in a NOC.Routing logic implemented as a network of synchronous and asynchronouslogic can be configured to make routing decisions as fast as a singleclock cycle. The routing logic in this example routes packets byselecting a port for forwarding each packet received in a router. Eachpacket contains a network address to which the packet is to be routed.

In describing memory-address-based communications above, each memoryaddress was described as mapped by network interface controllers to anetwork address, a network location of a memory communicationscontroller. The network location of a memory communication controller106 is naturally also the network location of that memory communicationcontroller's associated router 110, network interface controller 108,and IP block 104. In inter-IP block, or network-address-basedcommunications, therefore, it is also typical for application-level dataprocessing to view network addresses as the location of an IP blockwithin the network formed by the routers, links, and bus wires of theNOC. FIG. 2 illustrates that one organization of such a network is amesh of rows and columns in which each network address can beimplemented, for example, as either a unique identifier for each set ofassociated router, IP block, memory communications controller, andnetwork interface controller of the mesh or x, y coordinates of eachsuch set in the mesh.

In NOC 102 of FIG. 3, each router 110 implements two or more virtualcommunications channels, where each virtual communications channel ischaracterized by a communication type. Communication instruction types,and therefore virtual channel types, include those mentioned above:inter-IP block network-address-based messages, request messages,responses to request messages, invalidate messages directed to caches;memory load and store messages; and responses to memory load messages,and so on. In support of virtual channels, each router 110 in theexample of FIG. 3 also includes virtual channel control logic 154 andvirtual channel buffers 156. The virtual channel control logic 154examines each received packet for its assigned communications type andplaces each packet in an outgoing virtual channel buffer for thatcommunications type for transmission through a port to a neighboringrouter on the NOC.

Each virtual channel buffer 156 has finite storage space. When manypackets are received in a short period of time, a virtual channel buffercan fill up—so that no more packets can be put in the buffer. In otherprotocols, packets arriving on a virtual channel whose buffer is fullwould be dropped. Each virtual channel buffer 156 in this example,however, is enabled with control signals of the bus wires to advisesurrounding routers through the virtual channel control logic to suspendtransmission in a virtual channel, that is, suspend transmission ofpackets of a particular communications type. When one virtual channel isso suspended, all other virtual channels are unaffected—and can continueto operate at full capacity. The control signals are wired all the wayback through each router to each router's associated network interfacecontroller 108. Each network interface controller is configured to, uponreceipt of such a signal, refuse to accept, from its associated memorycommunications controller 106 or from its associated IP block 104,communications instructions for the suspended virtual channel. In thisway, suspension of a virtual channel affects all the hardware thatimplements the virtual channel, all the way back up to the originatingIP blocks.

One effect of suspending packet transmissions in a virtual channel isthat no packets are ever dropped. When a router encounters a situationin which a packet might be dropped in some unreliable protocol such as,for example, the Internet Protocol, the routers in the example of FIG. 3may suspend by their virtual channel buffers 156 and their virtualchannel control logic 154 all transmissions of packets in a virtualchannel until buffer space is again available, eliminating any need todrop packets. The NOC of FIG. 3, therefore, may implement highlyreliable network communications protocols with an extremely thin layerof hardware.

The example NOC of FIG. 3 may also be configured to maintain cachecoherency between both on-chip and off-chip memory caches. Each NOC cansupport multiple caches each of which operates against the sameunderlying memory address space. For example, caches may be controlledby IP blocks, by memory communications controllers, or by cachecontrollers external to the NOC. Either of the on-chip memories 114, 116in the example of FIG. 2 may also be implemented as an on-chip cache,and, within the scope of the present invention, cache memory can beimplemented off-chip also.

Each router 110 illustrated in FIG. 3 includes five ports, four ports158A-D connected through bus wires 118 to other routers and a fifth port160 connecting each router to its associated IP block 104 through anetwork interface controller 108 and a memory communications controller106. As can be seen from the illustrations in FIGS. 2 and 3, the routers110 and the links 118 of the NOC 102 form a mesh network with verticaland horizontal links connecting vertical and horizontal ports in eachrouter. In the illustration of FIG. 3, for example, ports 158A, 158C and160 are termed vertical ports, and ports 1588 and 158D are termedhorizontal ports.

FIG. 4 next illustrates in another manner one exemplary implementationof an IP block 104 consistent with the invention, implemented as aprocessing element partitioned into an issue or instruction unit (IU)162, execution unit (XU) 164 and auxiliary execution unit (AXU) 166. Inthe illustrated implementation, IU 162 includes a plurality ofinstruction buffers 168 that receive instructions from an L1 instructioncache (iCACHE) 170. Each instruction buffer 168 is dedicated to one of aplurality, e.g., four, symmetric multithreaded (SMT) hardware threads.An effective-to-real translation unit (iERAT) 172 is coupled to iCACHE170, and is used to translate instruction fetch requests from aplurality of thread fetch sequencers 174 into real addresses forretrieval of instructions from lower order memory. Each thread fetchsequencer 174 is dedicated to a particular hardware thread, and is usedto ensure that instructions to be executed by the associated thread isfetched into the iCACHE for dispatch to the appropriate execution unit.As also shown in FIG. 4, instructions fetched into instruction buffer168 may also be monitored by branch prediction logic 176, which provideshints to each thread fetch sequencer 174 to minimize instruction cachemisses resulting from branches in executing threads.

IU 162 also includes a dependency/issue logic block 178 dedicated toeach hardware thread, and configured to resolve dependencies and controlthe issue of instructions from instruction buffer 168 to XU 164. Inaddition, in the illustrated embodiment, separate dependency/issue logic180 is provided in AXU 166, thus enabling separate instructions to beconcurrently issued by different threads to XU 164 and AXU 166. In analternative embodiment, logic 180 may be disposed in IU 162, or may beomitted in its entirety, such that logic 178 issues instructions to AXU166.

XU 164 is implemented as a fixed point execution unit, including a setof general purpose registers (GPR's) 182 coupled to fixed point logic184, branch logic 186 and load/store logic 188. Load/store logic 188 iscoupled to an L1 data cache (dCACHE) 190, with effective to realtranslation provided by dERAT logic 192. XU 164 may be configured toimplement practically any instruction set, e.g., all or a portion of a32 b or 64 b PowerPC instruction set.

AXU 166 operates as an auxiliary execution unit including dedicateddependency/issue logic 180 along with one or more execution blocks 194.AXU 166 may include any number of execution blocks, and may implementpractically any type of execution unit, e.g., a floating point unit, orone or more specialized execution units such as encryption/decryptionunits, coprocessors, vector processing units, graphics processing units,XML processing units, etc. In the illustrated embodiment, AXU 166includes a high speed auxiliary interface to XU 164, e.g., to supportdirect moves between AXU architected state and XU architected state.

Communication with IP block 104 may be managed in the manner discussedabove in connection with FIG. 2, via network interface controller 108coupled to NOC 102. Address-based communication, e.g., to access L2cache memory, may be provided, along with message-based communication.For example, each IP block 104 may include a dedicated in box and/or outbox in order to handle inter-node communications between IP blocks.

Embodiments of the present invention may be implemented within thehardware and software environment described above in connection withFIGS. 1-4. However, it will be appreciated by one of ordinary skill inthe art having the benefit of the instant disclosure that the inventionmay be implemented in a multitude of different environments, and thatother modifications may be made to the aforementioned hardware andsoftware embodiment without departing from the spirit and scope of theinvention. As such, the invention is not limited to the particularhardware and software environment disclosed herein.

Extensible Execution Unit Interface Architecture

Embodiments consistent with the invention utilize an extensibleexecution unit interface architecture that tightly couples the decodelogic of multiple execution units in a processing unit and supports theability for one execution to overlay one or more instructions of anotherexecution unit. In some embodiments, for example, priorities (which maybe hard-coded into a design) are assigned to different execution unitsand at least a portion of the instructions in an instruction stream areconcurrently forwarded to decode logic associated with multipleexecution units such that whenever an instruction is decoded as being avalid instruction for multiple execution units, the instruction will beforwarded along for execution only by the execution unit having thehighest priority.

In the embodiments discussed hereinafter, for example, an extensibleexecution unit interface architecture may be tightly coupled with atleast the decode, rename, execute and commit stages of a processing unitto maintain performance and encourage design reuse to drive down cost.The interface may be designed to be able to be used by a wide variety ofinstruction architecture additions, such as scalar floating point,vector fixed point, vector floating point, etc. The interface alsoallows an auxiliary execution unit (hereafter AXU) to “overlay” anexisting instruction that would normally be executed on the generalpurpose side of a processing unit, thus allowing, for example, adesigner, or even a customer, to replace an existing instruction withits own, allowing greater flexibility. As will also become more apparentbelow, overlaying an instruction may also be used to provide enhancedperformance for specialized applications, as well as to “patch”instructions that may be executed in a sub-optimal manner in otherexecution units.

By way of example, FIG. 5 illustrates a portion of an example processingunit 200, which may be implemented, for example, as an IP block from thecomputer of FIGS. 1-4. Processing unit 200 includes instruction logic202 implementing an extensible execution unit interface architecture andconfigured to supply instructions to one or more execution units, e.g.,execution units 204, 206, which are also designated as XU0 and XU1. Eachexecution unit may be implemented using any type of execution unit,e.g., a scalar or vector fixed point execution unit, a scalar or vectorfloating point execution unit, various types of accelerators orspecialized execution units (e.g., encryption/decryption engines, DMAengines, compression/decompression engines, physics engines, graphicsprocessors, coprocessors, etc.) Moreover, multiple instances of anexecution type may be supported in a given processing unit design, suchthat, for example, instructions may be forwarded to different executionunits of a given type of execution unit to increase overall throughput.

In addition, within the context of the invention, execution unitssharing the same “type” are capable of executing the same instructionssuch that a valid instruction decoded by the decode logic can beexecuted by all execution units of that type. The execution units of thesame type need not be configured identically to one another from eithera functionality or hardware perspective in some embodiments; however,they typically are configured such that they are all capable ofreceiving instructions from the same decode logic.

In this example embodiment, two-way multithreading is supported, and assuch up to two instructions are provided per cycle to a pair of threads210, 212 (also designated as Thread0 and Thread1). For each thread 210,212, one or more instruction processing stages are included, with onlythe decode and dependency stages illustrated in FIG. 5 to simplify thediscussion.

To implement an extensible execution unit interface architectureconsistent with the invention, the decode stage of each thread 210, 212includes decode logic 214, 216 respectively associated with each type ofexecution unit (i.e., execution units XU0 and XU1). The dependency stageof each thread 210, 212 similarly includes dependency logic 218, 220respectively associated with each type of execution unit. In addition,decode logic 214, 216 are in communication with one another to enableone or both of decode logic 214, 216 to assert an invalidate signal tothe other decode logic 214, 216 to invalidate the instruction in theother decode logic so that the instruction is not executed by anexecution unit associated with the other decode logic. Thread1 212includes similar decode logic 214, 216 and dependency logic 218, 220 asThread0 210 to process instructions associated with Thread1.

Each execution unit 204, 206 is coupled to an associated issue unit 222,224 and each issue unit 222, 224 is configured to receive instructionsfor the associated execution unit 204, 206 from the dependency logic218, 220 in each thread 210, 212 and associated with the respectiveexecution unit 204, 206. As such, issue units 222 arbitrate betweenthreads 210, 212 and pass instructions released by the dependency logicfrom each thread to the associated execution units for executionthereby.

Processing unit 200 includes an instruction set architecture (ISA) thatdefines the format of each instruction capable of being executed by theprocessing unit. However, in many embodiments, specific execution unitsare only capable of executing a subset of the instructions defined inthe ISA (e.g., a floating point unit may only handle floating pointinstructions), such that any instruction that is not capable of beingexecuted by a particular execution unit will be decoded as an invalidinstruction. Therefore, in the illustrated embodiment, instructionsassociated with a particular thread 210, 212 are concurrently forwardedto decode logic 214 and 216 and independently decoded by each suchdecode logic, such that for any instruction that is specific to aparticular execution unit will only be decoded as a valid instruction bythe decode logic for that execution unit. In addition, however, forinstructions that are capable of being executed by both execution units,the relative priorities of the execution units are used to control whichexecution unit will execute the instruction.

FIG. 6, for example, illustrates a state table defining the actions thatoccur based upon how each decode logic 214, 216 decodes a particularinstruction, and in the case where execution unit 206 (XU1) is assigneda higher priority than execution unit 204 (XU0). Where only one decodelogic 214, 216 decodes an instruction as a valid instruction, theassociated execution unit 204, 206 will execute the instruction.However, where both decode logic 214, 216 decode an instruction asvalid, the instruction will be executed by execution unit 206 (XU1)based upon its higher priority. In addition, in the case where neitherdecode logic 214, 216 decodes the instruction as a valid instruction, anillegal instruction program interrupt is signaled, which typicallyresults in the instruction being handled in software.

Priorities may be assigned in a number of manners consistent with theinvention. For example, priorities may be configurable at runtime via aspecial purpose register or other software-based control. Priorities mayalso be configurable based on a fuse, a control signal or other run-timebased mechanism. In the illustrated embodiments, however, priorities aretypically hard-coded and static in nature and implemented directlywithin the design of the processing unit. For example, in one embodimentdecode logic 216 associated with execution unit 206 may be configured tosend an invalidate signal to decode logic 214 in response to decoding avalid instruction, such that decode logic 214 invalidates theinstruction regardless of whether the instruction is decoded as valid orinvalid by decode logic 214. It will be appreciated, however, that othermanners of implementing a priority mechanism to control which executionunit executes an instruction that is decoded as a valid instruction bymultiple decode logic may be used, and that such alternateimplementations would be apparent to one of ordinary skill in the arthaving the benefit of the instant disclosure.

It will be appreciated that the aforementioned priority mechanism mayalso be extended to handle processing unit designs with more than twotypes of execution units. FIG. 7, for example, illustrates a state tablefor use with a processing unit having three types of execution unitsXU0, XU1 and XU2, where execution unit XU2 has the highest priority,followed by execution unit XU1 and then execution unit XU0. As shown inthe table, whenever the decode logic associated with execution unit XU2decodes a valid instruction, the instruction will always be executed bythat execution unit, but where execution unit XU2 decodes an instructionas an invalid instruction, execution unit XU1 will still be prioritizedover execution unit XU0 in the same manner as discussed above inconnection with FIG. 6.

To further illustrate various applications of an extensible executionunit interface architecture consistent with the invention, FIG. 8 nextillustrates a processing unit 250 incorporating a general core 252including an issue unit 254, reservation unit 256, first and secondfixed point execution units 258, 260 (FX0, FX1), a load store unit 262(LQ, with data cache) and memory management unit 264. In this example,general core 250 is a general purpose processing core that is capable ofbeing integrated with various auxiliary execution units (AXU's) tocreate a specialized application-specific processing unit. For example,in the application shown in FIG. 8, general core 250 may be integratedwith a scalar double precision floating point AXU 266 to provideenhanced floating point capabilities for the processing unit.

Issue unit 254 includes various instruction fetching componentscollectively illustrated in block 268, e.g., an instruction fetch unit,branch prediction logic, an instruction cache, a microcode unit and aninstruction buffer. Multithreaded instruction logic 270 receives up totwo instructions (instr0, instr1) for two threads per cycle, and eachinstruction is passed to paired decode logic 272, 274 respectivelyassociated with a fixed point execution unit type (FXU) and an auxiliaryexecution unit type (AXU). Paired rename logic 276, 278 respectivelyassociated with the FXU and AXU execution unit types receiveinstructions from the decode logic 272, 274 and apply renamingoperations to the instructions to map the operands to registers in theregister files for the associated execution units based upon therespective register file architectures for the execution units.Completion logic 280 is used to coordinate the issuance of instructionsamong the multiple threads, and to retire instructions upon completionof execution.

Reservation logic 282 includes reservation stations 282, 284, 286 and288 respectively associated with FX0 execution unit 258, FX1 executionunit 260, load store unit 262 and AXU 266, which respectively queueinstructions for execution by the associated execution unit.

In operation, two instructions, instr0 and instr1, are concurrentlyforwarded to the FXU decoder logic 272 and AXU decoder logic 274. TheFXU decoder logic typically decodes all general purpose instructionssupported by the processing unit architecture, such as branches, fixedpoint math, fixed point loads and stores, cache management instructions,etc. The AXU decoder logic unit is primarily directed to execute doubleprecision scalar floating point instructions, but may be configured tosupport one or more of the instructions also supported by FXU decoderlogic 272.

If FXU decoder logic 272 decodes an instruction as one of its own, andAXU decoder logic 274 does not, the instruction is considered a validFXU instruction and is executed on either the FX0 or FX1 fixed pointexecution units 258, 260. If AXU decoder logic 274 decodes aninstruction as one of its own and FXU decoder logic 272 doesn't, theinstruction is considered a valid AXU instruction and is executed by AXU266. It should be noted that the interface may support multiple AXU'sthat may be identical to one another such that instructions may beissued to the first available AXU, or may be different to supportdifferent instruction set architectures.

If both AXU decoder logic 274 and FXU decoder logic 272 decode theinstruction as a valid instruction, AXU 266 will execute the instructionso that, in effect, AXU 266 replaces the functionality of an existinginstruction. If no decode logic decodes the instruction as its own, theinstruction is considered to be illegal and an illegal instructionprogram interrupt occurs.

Once the two instructions for the two threads are decoded, theinstructions proceed to rename logic 276, 278 in a rename stage, whichtranslates the architected register addresses identified in theinstructions to corresponding physical register addresses. This allowseach instruction to operate on a larger pool of physical addresses,allowing more instructions to be issued and executed out of order, whichimproves performance. In addition, if a particular AXU implementationdoes not need to rename its registers, or doing so would actually hamperperformance, the AXU rename logic may simply set the physical registernumber to the architected register number, effectively bypassing anyregister renaming. Each rename logic 276, 278 also interfaces withcompletion logic 280, as once instructions are completed, the associatedrename physical registers are freed up if not needed. After the physicaladdresses are determined by rename logic 276, 278, the instructionsproceed to the reservation stations 282-288, which track dependenciesbetween instructions, and holds off on the issuance of dependentinstructions until any required data is ready. In many implementations,instructions may be forwarded to the associated execution units in anout of order fashion due to outstanding dependencies.

Various additional logic may be utilized in processing unit 250 tosupport an extensible execution unit interface architecture. Forexample, shared registers, such as the condition register (CR), whichare shared between execution units often and require high performance,may also be renamed to an address that is in a bigger pool than thearchitected size, such that AXU instructions that alter the CR don'tnecessarily hamper performance for FXU instructions. Further, to supportmultithreading, a thread ID signal may be provided with eachinstruction. If a particular AXU supports fewer threads than what existsin the microprocessor for the FXU, and the AXU is sent a thread id thatis higher than what it supports, it will cause a Unimplementedinstruction interrupt. In addition, for the purpose of loads and stores,separate AXU LQ decode logic 290 may be used to enable the AXU toprovide the load store unit (LQ) 262 with a set of signals that describea load or store in generic terms, such as how many bytes to load/store,how to calculate the memory address, the endian-ness, etc, such that LQ262 will return load data to the AXU register file and bypass, andreceive store data from the AXU register file.

In addition, in some embodiments, it may be desirable to utilize aninterface to an AXU that supports indefinite or indeterminate latencies,such that an AXU is required to return a signal to issue unit 254 withan instruction tag and thread ID when it has finished executing aparticular instruction. As such, while instructions may be concurrentlyforwarded to multiple decode logic associated with different executionunit types, it is not always the case that the respective decode logicdecode an instruction precisely simultaneously, or that other stages ofthe instruction processing pipeline occur simultaneously for everyexecution unit.

An extensible execution unit interface architecture consistent with theinvention provides a number of benefits, particularly in the developmentof application specific hardware designs. For example, it will beappreciated that while AXU decode logic 274, AXU rename logic 278 andother supporting circuitry for the AXU are illustrated within generalcore 252, such logic in many cases may be physically disposed outside ofthe core and disposed within AXU 266 or elsewhere on an integratedcircuit. Particularly in the case where general core 252 is a designed,verified and tested core, it may be desirable to simply provide aninterface to AXU decode logic 274 and any other AXU-related logic suchthat such logic may be added to an application-specific processing unitdesign without having to modify the design of the general core, andthereby enabling the general core to be used in a wide variety ofapplications. As such, a general core may be integrated with additionalfunctionality simply by adding an execution unit and associated decodeand other instruction logic to a processing unit design, and interfacedwith appropriate interfaces in the general core.

One benefit of such a configuration is that different parties cancombine their respective intellectual property without the need todisclose such intellectual property to one another. A party wishing toincorporate their own proprietary accelerator functionality, forexample, could rely on another party's general core for the remainder ofthe required functionality, and thereby limit their custom design,testing and verification to the AXU, often saving considerable cost andeffort associated with development of an application specific hardwaredesign. In addition, if one party needs to limit access to theirconfidential intellectual property, e.g., due to its proprietary nature,or due to government confidential or classified designs, that party maydo so without having to disclose such confidential information to theother party.

Furthermore, the illustrated embodiments allow for higher performanceand/or greater flexibility and quick design turnaround and theflexibility for a chip designer or customer to redefine an existinginstruction in an established instruction set architecture. As notedabove, the redefinition may be used to provide enhanced performance foran instruction through the use of a specialized execution unit, or maybe used to correct for a design flaw in core that results in sub-optimalexecution of the instruction in the core.

It will be appreciated that a wide variety of alternate configurationsmay be utilized to that illustrated in FIGS. 5 and 8. For example,various alternative issue logic configurations may be used, e.g., wherevarious components or stages are combined for different execution unitsor threads, rather than being implemented separately, or whereadditional and/or alternative components or stages are used. Differentmemory architectures and cache structures may be used in otherembodiments, as may different numbers and combinations of executionunits.

Various additional modifications may be made to the disclosedembodiments without departing from the spirit and scope of theinvention. Therefore, the invention lies in the claims hereinafterappended.

What is claimed is:
 1. A circuit arrangement, comprising: first decodelogic associated with a first type of execution unit and configured toreceive an instruction and decode the instruction; and second decodelogic associated with a second type of execution unit and configured toreceive the instruction and decode the instruction, wherein theinstruction is concurrently forwarded to the first and second decodelogic; wherein the first decode logic has a higher priority than thesecond decode logic such that if the first and second decode logicrespectively decode the instruction as a valid instruction for the firstand second types of execution unit, the instruction is executed by afirst execution unit of the first type of execution unit rather a secondexecution unit of the second type of execution unit.
 2. The circuitarrangement of claim 1, wherein the first and second decode logic areconfigured such that: in response to the first decode logic decoding theinstruction as a valid instruction for the first type of execution unitand the second decode logic decoding the instruction as an invalidinstruction for the second type of execution unit, the instruction isforwarded to the first execution unit for execution thereby; in responseto the first decode logic decoding the instruction as an invalidinstruction for the first type of execution unit and the second decodelogic decoding the instruction as a valid instruction for the secondtype of execution unit, the instruction is forwarded to the secondexecution for execution thereby; and in response to the first decodelogic decoding the instruction as a valid instruction for the first typeof execution unit and the second decode logic decoding the instructionas a valid instruction for the second type of execution unit, theinstruction is forwarded to the first execution unit for executionthereby.
 3. The circuit arrangement of claim 2, wherein the first andsecond decode logic are further configured such that in response to thefirst decode logic decoding the instruction as an invalid instructionfor the first type of execution unit and the second decode logicdecoding the instruction as an invalid instruction for the second typeof execution unit, an interrupt is signaled.
 4. The circuit arrangementof claim 3, wherein the interrupt is an illegal instruction programinterrupt.
 5. The circuit arrangement of claim 1, wherein the firstdecode logic is configured to assert an invalidate signal to the seconddecode logic in response to decoding the instruction as a validinstruction for the first type of execution unit such that theinstruction is invalidated by the second decode logic.
 6. The circuitarrangement of claim 1, further comprising the first and secondexecution units.
 7. The circuit arrangement of claim 6, furthercomprising a general core, wherein the second decode logic and thesecond execution unit are disposed in the general core, and wherein thefirst decode logic and the first execution unit are external to thegeneral core.
 8. The circuit arrangement of claim 7, wherein the firstand second execution units are each configured to execute a firstinstruction, and wherein the first execution unit is configured toexecute the first instruction with greater performance than the secondexecution unit such that the first execution unit optimizes the generalcore for execution of the first instruction.
 9. The circuit arrangementof claim 7, wherein the first and second execution units are eachconfigured to execute a first instruction, wherein the general coreincludes a design flaw that causes the second execution unit to executethe first instruction with sub-optimal performance, and wherein thefirst execution unit is configured to execute the first instruction withgreater performance than the second execution unit such that the firstexecution unit corrects for the design flaw in the general core.
 10. Thecircuit arrangement of claim 1, further comprising: third decode logicassociated with a third type of execution unit and configured to receivethe instruction concurrently with the first and second decode logic anddecode the instruction; wherein the first and second decode logic eachhave a higher priority than the third decode logic such that if eitherof the first and second decode logic respectively decode the instructionas a valid instruction for the first or second type of execution unit,the instruction is executed by the first or second execution unit rathera third execution unit of the third type of execution unit.
 11. Thecircuit arrangement of claim 1, further comprising first and secondrename logic respectively associated with the first and second decodelogic, wherein the first and second execution units have differentregister architectures, and wherein the first and second rename logicare configured to apply different renaming operations to the instructionbased upon the respective register architectures of the first and secondexecution units.
 12. The circuit arrangement of claim 1, furthercomprising: a load store unit; and load store decode logic coupled tothe load store unit and configured to decode load and store instructionsassociated with the first execution unit.
 13. The circuit arrangement ofclaim 1, wherein the first and second decode logic are disposed in amultithreaded processing unit, wherein each instruction is associatedwith a thread identifier, and wherein the instruction triggers anunimplemented instruction interrupt in response to the instruction beingforwarded to the first execution unit and having an invalid threadidentifier for the first execution unit.
 14. The circuit arrangement ofclaim 1, wherein the first execution unit comprises an auxiliaryexecution unit and the second execution unit comprises a fixed pointexecution unit.
 15. An integrated circuit device including the circuitarrangement of claim
 1. 16. A program product comprising a computerreadable storage medium and logic definition program code resident onthe computer readable storage medium and defining the circuitarrangement of claim 1.